The present invention relates to a liquid level sensor for detecting the liquid level in a chemical solution processing bath used for wet processes during manufacturing processes of semiconductors or liquid crystals or the like.
Manufacturing processes of semiconductors or liquid crystals involve wet processes such as wafer cleaning, wet etching, etc. Chemical solution baths of wet processing systems are equipped with various types of sensors to detect their liquid level.
For example, Japanese Patent Application No. 204206/94 describes a liquid level sensor in which a chemical solution bath 3 for processing semiconductor wafers 2 with a chemical solution is fed with air or N.sub.2 gas at a constant flow rate through a gas feed tube 6, as shown in FIG. 1. An end of the gas feed tube 6 is immersed in the chemical solution 1 to determine the liquid level based on the variation of the pressure of N.sub.2 or air reflecting the liquid level. Any pressure difference is detected by an air sensor 5 to deliver an electric signal, according to which a control means 4 controls the liquid level in the chemical solution bath 3 or the like. Thus, the liquid level sensor for wet processes serves to control the operation of the system by feeding air or N.sub.2 gas at a constant flow rate, detecting the pressure by the air sensor 5 and converting it into an electric signal to indicate the normal level during supply of the chemical solution to the chemical solution bath 3, or the lower limit level during discharging.
The liquid level sensor should operate precisely in wet process systems, because it sometimes serves to stabilize the process by detecting(g any drop in liquid level due to evaporation of the chemical solution to deliver a signal for adding the chemical solution. However, the above liquid level sensor sometimes erroneously operates when deposits 8 are formed on the inner wall of the end of the gas feed tube 6 from a process solution, such as ammonium fluoride (NH.sub.4 F) dissolved in buffered hydrofluoric acid (HF: NH.sub.4 F=1:5 to 1:500), or surfactant-added buffered hydrofluoric acid or pure water-added buffered hydrofluoric acid, or the like (hereinafter collectively referred to as buffered hydrofluoric acid), as shown in FIG. 2.
This can be explained as follows. If N.sub.2 is used as a gas, N.sub.2 gas 9 is flowed out from the end of the gas feed tube 6 in the form of bubbles into the chemical solution 1. As N.sub.2 gas 9 is released from the end of the gas feed tube 6, trace amounts of buffered hydrofluoric acid are deposited on the inner wall of the gas feed tube 6 and dried with high-purity N.sub.2 gas free from moisture so that ammonium fluoride dissolved in buffered hydrofluoric acid separates out. Thus, ammonium fluoride particles gradually grow up to finally block the gas feed tube 6. When the gas feed tube 6 is blocked by deposition of ammonium fluoride, N.sub.2 gas pressure rises so that the sensor 5 erroneously operates to send a signal indicative of a rise in liquid level to the wet system, which in return triggers an alarm, though the liquid level is normal.
In order to restart the wet system, ammonium fluoride deposits in the gas feed tube 6 must be removed. However, the removal operation requires that still usable buffered hydrofluoric acid be discharged because it is dangerous to remove the deposits while retaining the chemical solution 1 in the chemical solution bath 3.
In view of the above problems, the present invention aims to provide a liquid level sensor free from the erroneous operation as described above by preventing deposition of ammonium fluoride or the like.